This application relates to electro-mechanical actuators, particularly to an actuator formed from braided shape memory materials which provides an increased electrical resistance such that it is easier to actuate electrically.
Shape memory alloys (SMAs) and shape memory polymers (SMPs) have many unique material properties that allow them to be used as actuators. These properties include simplicity, low cost, high energy/weight ratio, and near silent operation of motion. Conventional SMA actuators, such as Nickel-Titanium NiTi, are simple straight wires that contract with large force when heated and relax when cooled and stressed—this is called the ‘shape memory effect’ [U.S. Pat. No. 3,463,238]. Using known alloys, actuators can be designed that are capable of exhibiting this motion many thousands of times without exhaustion when heated and cooled. Many shape memory materials have sufficient electrical resistivity that SMA wires can be heated by passing electric current through them, making them effective actuators.
Unfortunately the strain of a straight SMA wire is on the order of only 5%—this is very small compared to typical actuation needs; strains of ˜50% are more useful in most applications. One common technique to increase the strain of an SMA actuator is to form it into a coil-spring [U.S. Pat. No. 4,490,975, U.S. Pat. No. 4,586,335, U.S. Pat. No. 8,607,562, US2015/0073318]. The coil spring is formed by, for instance, winding a straight wire in a spiral around a mandrel, clamping against the mandrel, heat-treating in a furnace at high temperatures (350 to 600 C), and quenching in water. Subsequently the wire will retain the shape it was held in during heat-treatment—called the ‘memory’ shape. It will be easily deformed when cold, but will return to the memorized shape when subsequently heated. Because the material in such a coil-spring has primarily internal shear deformation, the extension and contraction of the device, when heated and cooled, can be as much as 300% of the device length. However, the strength of SMA coil-spring actuators is very small compared to that of the straight wires—large strain is achieved but the actuator stress capability is greatly diminished. It is often necessary to place multiple springs in parallel to achieve useful actuator strength, though this increases the size and complexity of the actuator [U.S. Pat. No. 4,553,393]. A problem with small coil-springs is that they are fragile and can be easily kinked or crushed during use.
Braiding is a textile formation process that involves the interlacing of three or more yarns. Typically braids are formed using a Maypole Braiding Machine which creates a tubular braid. The braided tube is often formed over a mandrel—an internal form that determines the final inner dimensions of the part.
Braids of shape memory materials, trained to remember a tubular form, have been used as medical stents which expand once upon warming into a super-elastic state [U.S. Pat. No. 3,868,956].
Electrically insulating coatings have been designed that can accommodate the large material strain of shape memory materials without cracking. Insulating methods in the literature include coating the surface of the wires with an insulating material such as Polytetrafluoroethylene or polyether-ether-ketone [Sheiko et al, Applied Surface Science 289 (2016) 651-665], and by intentional oxidation of the surface [U.S. Pat. No. 6,410,886].